Process for producing a leather substitute



United States Patent 3,494,781 PROCESS FOR PRODUCING A LEATHER SUBSTITUTE David E. Knibbe and John G. J. Kelly, Delft, Netherlands, assignors t0 Shell Oil Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 20, 1967, Ser. No. 684,534 Claims priority, application Netherlands, Dec. 20, 1966, 6617922 Int. Cl. D06n 3/00 U.S. Cl. 117-63 5 Claims ABSTRACT OF THE DISCLOSURE Synthetic leather articles are prepared by impregnating a nonwoven fabric with a prevulcanized polyisoprene latex, coagulating the polyisoprene on the fabric, washing, drying and then coating the impregnated fabric to form the leather substitute.

Leather-like products in the form of sheets consisting of layers of macromolecular plastics reinforced by webs composed of synthetic fibers are already known and have been more or less satisfactory as substitutes for leather.

Especially if the artificial leather is to be used for the manufacture of shoes many very stringent requirements are set. In this case the material should be moistureabsorbent and pervious to water vapor. On the other hand it should be impervious to liquid water and even be water-repellent. The various layers should form a solid bond. The material should in every respect lend itself well to stretching operations during the manufacture of shoes, in particular of toe and heel sections. The material should further be scratch-proof, feel like leather and otherwise meet decorative surface requirements.

It has now been found that the many requirements can be preeminently met by using prevulcanized polyisoprene in the form of a latex, close attention moreover being paid to a number of varying conditions specified hereinafter.

The invention may be defined as relating to a process for producing an artificial leather, characterized in that a web or a felt layer weighing between 100 and 250 g./m. and consisting of synthetic fibers with a length between 1 and cm. and less than 10 denier, is impregnated with a latex of synthetic polyisoprene having a cis content of more than 80%, which latex has been subjected to a prevulcanized treatment and contains sodium silicofluoride as coagulant, the weight ration of synthetic fibers to polyisoprene lying between 1:1 and 1:3, that the latex on the web is subsequently coagulated by applying heat in combination with pressures between 3 and 50 kg./cm. care being taken to prevent the water present from boiling, and that finally the impregnated web is washed with water, dried and provided with a coating for greater wear-resistance and/or improved finish.

The felt layer used as starting material is composed of fibers hanging loosely together. Webs (felt) layers of this type are known as nonwoven fabrics. If necessary, the coherence of the fibers can be further improved by treating the web with forked needles which, moving up and down, repeatedly pierce the web and in doing so rotate on their axis. Such a treatment is also useful for lessening the orientation of the fibers if there should be any orientation present.

It may be desirable to use webs or felt layers in which the fibers are interconnected at mutual points of contact either by means of a superficial fusion, which may be obtained by heating, or by means of an adhesive.

Suitable synthetic fibers are those of polyolefins, polyamides, such as nylon, polypivalolactone and poly- 3,494,781 Patented Feb. 10, 1970 esters, such as polymers of acrylic acid and methacrylic acid esters and polyesters based on terephthalic acid or isophthalic acid. Preference is accorded to fibers of polypropylene.

The preparation of synthetic polyisoprene with a cis content of more than and the preparation of latices from this polymer are well-known techniques. The cis content is determined from the infrared spectrum. A latex containing the polyisoprene in vulcanized form can be prepared by adding to the latex vulcanizing agents and possibly accelerators and/ or activators, and by a heat treatment. vulcanizing agents commonly used are sulfur or sulfur compounds. Suitable accelerators are, for example, Zinc dialkyldithiocarbamate and zinc mercaptobenzothiazole. As an activator, use is often made of zinc oxide.

Suitable concentrations of the polyisoprene in the latex lie in particular between 50 and 70% by weight.

Suitable quantities of sodium silicofluoride lie between 0.1 and 3% by weight, based on the polyisoprene. This is preferably added to the latex subsequent to the prevulcanization treatment.

The impregnation can be carried out by spraying or, alternatively, by wetting or soaking. The homogeneous distribution of the latex over the fibers can be promoted by applying light pressure, for example, 0.1 to 1 atm., before coagulation takes place.

During the heating of the impregnated layer, as a result of which coagulation takes place, temperatures between 80 C. and 110 C., for example, approximately 100 C., are normally used. At the same time the layer is subjected to a pressure between 3 and 50 kg./cm. The conditions selected are such that the water does not come to the boil. An apparatus suitable for heating purposes is a hot, rotating drum, over about half the circumference of which a belt moves which is made of leather, rubber or metal or combinations of these materials. The impregnated layer is then passed between the surface of the drum and the belt, and forced against the drum by this belt. The speed of rotation is set in such a manner that the heating time is long enough to ensure a suflicient degree of coagulation. The heating time is preferably in the order of 10300 seconds. Rarely will the residence time between drum surface and belt exceed 30 seconds.

The prevulcanized polyisoprene with a cis 1,4-content of more than 80% (preferably -97%) has excellent flow properties, as a result of which it coalesces well during coagulation and permits a highly uniform distribution over the fibers. This produces a uniform, fine porosity, which is of the utmost importance in view of the use of the product 'for the manufacture of shoes and the relevant requirement of waterproofness combined with permeability to water vapor.

It is preferred that the latex be prevulcanized by the heating with a sulfur containing vulcanizing agent and an accelerator to form a product having a cross link density in the order of from 0.01 10 to 0.25 10" This may be achieved by heating the latex with, for example, 0.25-25 phr. of sulfur and 0.252.5 phr., of a thiocar-bamate (e.g., zinc dialkylthiocarbamate) for 5 hours at 4070 C. Cross link densities may be calculated according to the Flory-Rehner equation as described in J. Chem. Phys. 11 521 (1943).

The impregnated layer is subsequently washed with water to remove water-soluble impurities. After this the layer is dried, preferably in hot air. In some cases the vulcanization proceeds further during the treatments in which heat is applied.

Finally, a treatment follows in which the product is provided either on one side or on both sides with a film, the purpose of which is to make the product feel more like leather and/or to give it an improved scratch and wear resistance. Pre-eminently suitable for this purpose are polyvinyl chloride and polyurethanes. Various copolymers or vinyl chloride, fOr example, with vinylidene chloride, can also be used. Such films can be applied by means of techniques known per se. It is possible to apply a film on the sheeted product by calendering or rolling them together at such an elevated temperature that plasticizing occurs with the result that the layers bond together. Polyvinyl chloride can be used as a paste in which it is in admixture with a plasticizer. If necessary, an intermediate layer forming a solid bond with the two materials can be applied in the form of a film or a liquid adhesive. In addition to the method described polyurethanes can also be applied to the sheeted product by the combined spraying of mixtures of diisocyanates and polyols,, in which case the desired outer film is formed in situ.

EXAMPLE I Synthetic non-woven 'felt:

Material Polypropylene Average length of fibers cm Denier value of fibers 5 Weight of web g./m. 165 Polyisoprene Instrinsic viscosity before vulcanization (measured at 25 C. in toluene) dl./g 8.1 Cis content percent 92.4 Formulation in the latex:

Polyisoprene content (by weight) do 61.2 Sulfur phr 0.5 Zinc diethyldithiocarbamate phr 0.75

The latex was maintained at 50 C. for 21 hours to effect vulcanization. 2.0 phr. of sodium silicofluoride were then added.

The felt layer of polypropylene fibers was then impregnated by spraying one side with the latex, care being taken that the weight ratio of fibers to polyisoprene was 1:2.2.

The coagulation was effected at 100 C. under a pressure of 6 kg./cm. Use was made of a continuously operating vulcanizing mechanism with drum (dia. 2 m.) and belt, as described above. The sprayed side of the felt layer was laid against the belt. Heat was supplied by the drum. In this way the latex had an opportunity to spread through the felt layer before coagulation made this impossible. Heat and pressure were applied for 20 sec- EXAMPLE II The synthetic fiber material used in this example was nylon.

Average length of fibers c cm 4 Denier value of fibers 3 Weight of web g./m. 190

The latex was the same as in Example I.

The non-woven fiber sprayed with the latex on both sides, 420 g. of rubber being used per 190 g. of web. To ensure a uniform distribution of the latex the sprayed web was place between two steel plates and passed thus between the rubber-lined mill rolls of a wringer.

The impregnated layer was subsequently put in a press and subjected to a pressure of 10 kg./cm. for seconds at a temperature of 100 C.

Finally, the product was coated with a spreading paste or plastisol consisting of polyvinyl chloride and a plasticizer. Thus coated, the product was passed at C. through a calender, one roll of which was embossed so that a surface structure was obtained in the product.

EXAMPLE III A nylon web as described in Example II was sprayed on one side with latex. A polypropylene web as described in Example I was then applied to the same side, whereupon the laminate thus formed was sprayed with the latex on both sides. The latex was formulated as specified in Example I. The overall weight ratio of latex to fibers was 122.2.

The coagulation in the laminate was effected by means of a treatment as described in Example II, except that the treatment lasted 25 instead of 20 seconds.

The coating with polyvinyl chloride took place as in Example II.

We claim as our invention:

1. A process for the production of a leather substitute article comprising the steps:

(a) impregnating a synthetic fiber non-woven fabric weighing -250 g./m. with a prevulcanized polyisoprene latex, said polyisoprene having a cis 1,4- content of at least 80%, the latex containing 0.1-3% by Weight of sodium silicofluoride based on the polyisoprene, the weight ratio of felt to polyisoprene being from 1:1 to 1:3;

(b) applying a pressure between 3 and 50 kg./cm. to

the impregnated fabric while heating for 10-300 seconds at a temperature of 80-1l0 C. while avoiding boiling of water, whereby the polyisoprene coagulates on the surface of the fibers in the felt;

(c) removing water solubles by water washing;

(d) substantially drying the impregnated fabric; and

(e) coating at least one surface thereof with a filmforming polymer of the group consisting of vinyl chloride polymers and polyurethanes.

2. A process according to claim 1 wherein the fabric is a polypropylene felt composed of fibers having a length of 1-10 cm. and a size less than 10 denier.

3. A process according to claim 1 wherein the prevulcanized polyisoprene has a cross-link density from 0.01 X 10- to 0.25 X 10- mole per cc.

4. A process according to claim 3 wherein the polyisoprene is prevulcanized by heating for 5-100 hours at 40--70 C. in the presence of 0.25-2.5 phr. sulfur and 0.25-25 phr. zinc diethyldithiocarbamate.

5. A process according to claim 4 wherein the sodium silicofluoride is added to the latex subsequent to the pre vulcanization treatment.

References Cited UNITED STATES PATENTS 2,407,582 9/1946 Soday 117140 X 2,697,048 12/1954 Secrist 117--140 X 2,973,284 2/1961 Semegen 117--l38.8 X 3,034,927 5/1962 Fairclough et al 117140 3,039,913 6/1962 Merrill et al. 117-1388 X 3,262,805 7/1966 Aoki 117140 X 3,377,307 4/1968 Kolb et al 117163 X 3,384,502 5/1968 Japs 117-138.8 X 3,438,920 4/1969 Halper et al. s 117161 X 3,185,582 5/1965 Alegre 11765.2 X

WILLIAM D. MARTIN, Primary Examiner H. J. GWINNELL, Assistant Examiner U.S. c1. X.R. 

